In wireless communication systems such as systems based on LTE (Long Term Evolution) and LTE-A (LTE-Advanced), which are developed by 3GPP (Third Generation Partnership Project), and WiMAX (Worldwide Interoperability for Microwave Access), which is developed by IEEE (The Institute of Electrical and Electronics engineers), a base station and a terminal each include one or a plurality of transmit/receive antennas, and utilize, for example, MIMO (Multiple Input Multiple Output) technology to achieve high-speed data transmission.
In the wireless communication systems, the support of MU-MIMO (Multiple User MIMO) in which a plurality of terminals perform spatial multiplexing using the same frequency and time resources is being examined. The support of a CoMP (Cooperative Multipoint) transmission scheme in which a plurality of base stations cooperate with one another to perform interference coordination is also being examined. For example, a wireless communication system designed to use heterogeneous network deployment (HetNet) implemented by a wide-coverage macro base station, an RRH (Remote Radio Head) with coverage smaller than the macro base station, and so on is being examined.
In the above-described wireless communication systems, interference occurs if uplink signals (uplink data or uplink control information) transmitted from a plurality of terminals have the same characteristics. Interference also occurs if uplink reference signals transmitted from a plurality of terminals have the same characteristics. To mitigate or eliminate interference between demodulation reference signals (also referred to as DMRSs) transmitted from a plurality of terminals, for example, a method for orthogonalizing the demodulation reference signals has been proposed (NPL 1).